To draw glass tubing, gather a large blob of glass on a blowpipe, attach a second blowpipe to the opposite side, and have two workers walk apart while one blows gently to hold the bore open. The result is tubing 1-30 mm in diameter and 1-3 m long. Pulling faster gives thinner walls, a larger gather gives a larger diameter, and blowing harder enlarges the bore.
Part of Glass Blowing and Forming
Pulling molten glass into straight, uniform tubing for laboratory work.
Tube Is Drawn Because The Glass Volume Stays The Same While The Length Grows
A drawn tube is a conservation problem. The glass has a fixed volume, so a longer tube means a smaller cross-section: more length, thinner wall or smaller diameter. The carrier gives tubing of 1-30 mm diameter and 1-3 m long, and its rule of thumb follows from that conservation: faster pulling gives thinner walls, more glass gives a larger diameter, and blowing harder gives a larger bore.
What You Need
- Two blowpipes and two workers; one pulls at each end.
- A clear floor or a long straight run, at least as long as the tube you want.
- A large gather at working temperature; see Gathering.
- A marked tray or rack to lay the finished tube on, and the annealing oven.
Method: Drawing
Step 1: Gather a large blob of glass.
Step 2: Attach the second pipe to the opposite side.
Step 3: Walk apart. Both workers move away from each other, stretching the glass into a tube.
Step 4: Blow gently during the stretch to keep the bore.
Step 5: Lay the tube straight on a rack while it stiffens, then anneal.
Step 6: Cut into lengths by scoring with a file and snapping, as in Lampworking.
Worked Example
Take an illustrative tube of 8 mm outer diameter and 1 mm wall, so an inner diameter of 6 mm. Cross-section is π ÷ 4 × (8² − 6²) = 0.7854 × 28 = 22 mm², or 0.22 cm². Three metres is 300 cm, so the volume of glass is 0.22 × 300 = 66 cm³, which at a density of about 2.5 g/cm³ (recalled, not from the corpus) weighs about 165 g. A gather far larger than that makes the tube much longer than the 1-3 m the carrier gives, or a thicker one; the gather size is the thing to adjust, along with the pulling speed.
Common Mistakes
- Pulling too fast. Walls thin out and may collapse.
- Not blowing. The bore closes.
- Letting the tube hang. It sags into a curve while soft.
Safety
Hot glass does not look hot
A drawn tube lying on a rack falls below about 600 °C quickly and shows no glow. A freshly drawn tube lying on a bench looks identical to a cold one and burns if gripped.
Two people on a rope of hot glass
The workers walk apart with their backs to different hazards. Clear the walking line of tools and people, agree a signal for stopping, and wear eye protection; a tube that snaps whips hot glass.
What’s Next
- Learn next: Lampworking (Flameworking) — shaping the tubing
- Combine with: Laboratory Glassware — what the tubing becomes